The preferred embodiment relates to a system for positioning a control element, particularly a pneumatically operated final control valve such as a fast-shutoff valve. Such final control valves are preferably operated via a pneumatic pivoting actuator. In normal operation of a process engineering plant such as a petrochemical plant in which final control valves are integrated, the pneumatic pivoting actuator furnishes compressed air for positioning the final control valve in various opening positions, preferably in maintaining it fully open. In an emergency situation—for example, on a power outage of the process engineering plant—the pneumatic pivoting actuator is activated such that the compressed air is dumped which releases a spring integrated in the pneumatic pivoting actuator, resulting in the final control valve being fully closed.
It is known to provide such pneumatic control element systems with a positioner or position controller. A controller microcomputer thereof generates from a desired/actual value comparison a signal for positioning the control element as desired. Particularly safe operating positioning systems for pneumatically operated control elements can be defined such that in addition to the positioner an automatic limit signal transmitter or limit stop switch is provided which is mainly required to monitor whether a desired position, particularly a final position of the final control valve, has actually been attained.
Such a positioning system is known for example from WO2005/093532 A1 in which both a positioner and a limit signal transmitter are provided. The limit signal transmitter communicates via a conductor with the positioner in which an insulating unit is integrated. The limit signal transmitter has its own power supply and is provided with its own microcomputer and a position sensor to ensure reliably safe functioning. One disadvantage in this known positioning system is that the overall configuration of the limit signal transmitter and positioner is extremely complex and involves high investment costs.